1. Field of the Invention
The present invention relates to a display panel in which a light emitting element is formed over a substrate and is sandwiched between the substrate and a cover member. The invention also relates to a display module obtained by mounting IC to the display panel. In this specification, the display panel and the display module are called by a generic term, light emitting device. The present invention also relates to a method of driving the light emitting device and to electronic device using the light emitting device.
2. Description of the Related Art
Being self-luminous, light emitting elements eliminate the need for back light necessary in liquid crystal display devices (LCDs) and therefore can make thinner devices. In addition, light emitting elements have higher visibility and no limitation in terms of viewing angle, and these are the reasons for attention that light emitting devices using light emitting elements are receiving in recent years as display devices to replace CRTs and LCDs.
A light emitting element has a layer containing an organic compound that provides luminescence (electro luminescence) generated upon application of electric field (hereinafter referred to as organic compound layer), as well as an anode layer and a cathode layer. Luminescence provided by organic compounds is divided into light emission upon return to base state from singlet excitation (fluorescence) and light emission upon return to base state from triplet excitation (phosphorescence). One or both of the fluorescence and phosphorescence can be used in a light emitting device of the present invention.
All the layers that are provided between an anode and a cathode of the light emitting element are organic compound layers in this specification. Specifically, the organic compound layer includes a light emitting layer, a hole injection layer, an electron injection layer, a hole transporting layer, an electron transporting layer, etc. A basic structure of a light emitting element is a laminate of an anode, a light emitting layer, and a cathode layered in this order. The basic structure can be modified into a laminate of an anode, a hole injection layer, a light emitting layer, and a cathode layered in this order, or a laminate of an anode, a hole injection layer, a light emitting layer, an electron transporting layer, and a cathode layered in this order.
In this specification, making a light emitting element to emit light is expressed as driving the light emitting element. The light emitting element as defined herein is an element that is composed of an anode, an organic compound layer, and a cathode.
Methods of driving a light emitting device having a light emitting element are roughly divided into analog driving methods and digital driving methods. Digital driving is deemed more promising in view of transition from analog broadcasting to digital broadcasting in recent years since it enables the light emitting device to display an image using a digital video signal that carries image information as it is without converting the signal into an analog signal.
Among the driving methods that obtain gradation display by binary voltages of digital video signals, there are an area division driving method and a time division driving method.
The area division driving method is a driving method in which gradation display is obtained by dividing one pixel into a plurality of sub-pixels and driving the sub-pixels individually in accordance with digital video signals. In the area division driving method, one pixel has to be divided into plural sub-pixels and each sub-pixel has to have its own pixel electrode in order to drive the sub-pixels individually. The area division driving method is therefore inconvenient in that the pixel structure is complicated.
On the other hand, the time division driving method is a driving method in which lengths of time a pixel is lit are controlled to obtain gradation display. Specifically, one frame period is divided into a plurality of subt-frame periods. In each sub-frame period, to be lit or not is determined for the respective pixel in accordance with digital video signals. The accumulated lengths of sub-frame periods during which a pixel is lit with respect to the length of the entire sub-frame periods in one frame period determine the gradation of that pixel.
Generally, an organic compound layer has faster response speed than a liquid crystal and therefore a light emitting element is suitable for time division driving.
A pixel portion of a light emitting device has a plurality of pixels. A circuit diagram of a pixel 9004 in a common light emitting device is shown in FIG. 16.
The pixel 9004 has one of source signal lines (source signal line 9005), one of power supply lines (power supply line 9006), and one of gate signal lines (gate signal line 9007). The pixel 9004 also has a switching TFT 9008 and a current controlling TFT 9009.
The switching TFT 9008 has a gate electrode connected to the gate signal line 9007. The switching TFT 9008 also has a source region and a drain region one of which is connected to the source signal line 9005 and the other of which is connected to a gate electrode of the current controlling TFT 9009 and to a capacitor 9010. Each pixel in the pixel portion has one capacitor.
The capacitor 9010 is provided to hold the gate voltage (the difference in electric potential between the gate electrode and a source region) of the current controlling TFT 9009 when the switching TFT 9008 is OFF.
The current controlling TFT 9009 has a source region and a drain region one of which is connected to the power supply line 9006 and the other of which is connected to a light emitting element 9011. The power supply line 9006 is connected to the capacitor 9010.
The light emitting element 9011 is composed of an anode, a cathode, and an organic compound layer placed between the anode and the cathode. If the anode is in contact with the source region or the drain region of the current controlling TFT 9009, the anode serves as a pixel electrode whereas the cathode serves as an opposite electrode. On the other hand, the cathode serves as the pixel electrode whereas the anode serves as the opposite electrode if the cathode is in contact with the source region or the drain region of the current controlling TFT 9009.
The opposite electrode of the light emitting element 9011 receives a given electric potential (opposite electric potential). The power supply line 9006 receives a given electric potential (power supply electric potential). The power supply electric potential and the opposite electric potential are provided by a power source placed in an IC external to the display device.
The operation of the pixel shown in FIG. 16 is described next. The description on the operation of the light emitting device shown in FIG. 16 is given while distinguishing the operation during a writing period and the operation during a display period from each other.
In a writing period, the opposite electric potential and the power supply electric potential are kept at the same level. A selection signal is inputted to the gate signal line 9007 to turn the switching TFT 9008 ON. Of n bit digital signals carrying image information (hereinafter referred to as digital video signals) and inputted to the source signal line 9005, digital video signals equivalent to one bit are inputted to the gate electrode of the current controlling TFT 9009 through the switching TFT 9008. The digital video signals inputted to the gate electrode of the current controlling TFT 9009 contain information, which is ‘1’ or ‘0’ and is used to control switching of the current controlling TFT 9009.
When digital video signals equivalent to one bit are inputted to the gate electrode of the current controlling TFT 9009 in every pixel, the writing period is ended to start a display period.
In a display period, there is an electric potential difference between the opposite electric potential and the power supply electric potential. When inputted digital video signals turn the current controlling TFT 9009 OFF, the electric potential of the power supply line 9006 is not given to the pixel electrode of the light emitting element 9011 and therefore the light emitting element 9011 does not emit light. On the other hand, when the current controlling TFT 9009 is turned ON, the power supply electric potential is given to the pixel electrode of the light emitting element 9011 and the electric potential difference between the opposite electric potential and the power supply electric potential causes the light emitting element 9011 to emit light.
The above operation is conducted for each set of one bit digital video signals to alternate a writing period with a display period. The accumulated lengths of display periods during which a light emitting element of a pixel emits light determine the gradation of that pixel.
FIG. 17 shows points at which writing periods and display periods are started in one frame period in the light emitting device that has the pixel illustrated in FIG. 16. The axis of an abscissa indicates time and the axis of an ordinate indicates the luminance of the light emitting element of the pixel. To simplify the explanation, the device is driven using 4 bit digital video signals and the light emitting element emits light in all of the display periods in FIG. 17.
One frame period has four writing periods Ta1 to Ta4 and four display periods Tr1 to Tr4 in correspondence with each bit of 4 bit digital video signals.
The light emitting element 9011 does not emit light in a writing period. Therefore, the luminance of the light emitting element 9011 is 0 in all of the writing periods Ta1 to Ta4.
When the display periods Tr1 to Tr4 are started, the light emitting element 9011 of the pixel starts emitting light (luminous state). As the display periods Tr1 to Tr4 are ended, the light emitting element 9011 stops emitting light (nonluminous state).
FIG. 17 shows points (timings) of starting display periods and writing periods in one pixel. Considered here is a case in which the operation shown in FIG. 17 is conducted in every pixel of the pixel portion.
After the operation shown in FIG. 17 is completed in every pixel, a writing period is ended to start a display period. At this point, a current flows into the light emitting element 9011 in every pixel at once. Thus, the amount of current flowing in the power supply line 9006 is increased sharply.
Because of the wiring line resistance of the power supply line 9006, the demand for current to be supplied to the light emitting element 9011 of the pixel 9004 temporarily surpasses the ability of the power supply line and the light emitting element 9011 fails to reach the designed luminance early in the display period. In FIG. 17, the ideal luminance of the light emitting element 9011 is indicated by the solid line and the actual luminance thereof is indicated by the dotted line.
This phenomenon is not limited to the case where all of the pixels in the pixel portion emit light simultaneously but could happen when more than one pixels out of all the pixels in the pixel portion are to emit light concurrently, though to varying degrees.
In the driving method of FIG. 17 using 4 bit digital video signals, the end of a writing period followed by the start of a display period takes place four times in one frame period. Each time a writing period is ended to start a display period, the luminance of the light emitting element is lowered temporarily to cause an image on the screen to flicker.
One of other driving methods than the one shown in FIG. 17 is a driving method in which the power supply electric potential and the opposite electric potential are kept constant all the time. In this driving method, the light emitting element emits light not only in a display period but also in a writing period. Accordingly, if the current controlling TFT is always ON, light emitting elements of plural pixels do not start emitting light simultaneously.
However, depending on the gradation of pixels, even this driving method allows light emitting elements of plural pixels to simultaneously start emitting light, which may take place several times in one frame period. Each time the light emitting elements start emitting light simultaneously, the luminance of the light emitting elements is lowered temporarily.